...In written French, elision (both phonetic and orthographic) is obligatory for the following words:

• the definite articles le and la
• le garçon ("the boy"), la fille ("the girl")
• le + arbre → l'arbre ("the tree"), la + église → l'église ("the church")
• the subject pronouns je and ce (when they occur before the verb)
• Je dors. ("I sleep") Ce serait génial. ("That would be great.")
• J'ai dormi. ("I slept.") C'était génial. ("It was great.")
• but: Ai-je imaginé? ("Did I imagine?"), Est-ce utile? ("Is that useful?")
• the object pronouns me, te, se, le, and la (when they occur before the verb)
• Jean se rase, la voit, me téléphone. ("Jean shaves himself, sees her, phones me.")
• Jean s'est rasé, l'a vue, m'a téléphoné. ("Jean shaved himself, saw her, phoned me.")
• but: Regarde-le encore une fois. ("Look at him one more time.")
• the object pronouns le, la, moi, toi when they occur after an imperative verb and before the pronoun en or y:
• Mettez-le, donne-les-moi, casse-toi. ("Put it, give me them, scram.")
• Mettez-l'y, donne-m'en, va-t'en. ("Put it there, give me some, leave.")
• the negative marker ne
• Elle ne parle plus. ("She isn't talking anymore.")
• Elle n'arrête pas de parler. ("She won't stop talking.")
• the preposition de
• Le père de Jean vient de partir. ("Jean's father just left.")
• Le père d'Albert vient d'arriver. ("Albert's father just arrived.")
• que (which has many different functions)
• Que dis-tu ? Que Jean ne fait que manger. ("What are you saying? That Jean does nothing but eat.")
• Qu'as-tu dit ? Qu'il ne nous restait plus qu'une semaine. ("What did you say? That we only had one more week left.")
• The conjunction si plus the pronouns il and ils
• si elle aime les chats ("if she likes cats")
• s'il(s) aime(nt) les chats ("if he/they like cats")
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